Handle EWMH requests to change current desktop

Patch status: needinfo

Patch by Tony Crisci

Long description:

This request is used by pagers and bars to change the current
desktop likely as a result of some user action. We interpret this as
a request to focus the given workspace.
for more information see:
http://standards.freedesktop.org/wm-spec/latest/ar01s03.html#idm140251368135008